In general, the present invention relates to improvements in projection screens and other radiation-redistributive devices, such as lighting reflectors and refractors, illumination aids for photographic prints, traffic signs, advertisements, etc.
Heretofore, a wide variety of radiation-redistributive devices .[.has.]. .Iadd.have .Iaddend.been proposed to achieve such features as a definitely controllable field through which incident radiation is redistributed, uniform radiance throughout such field, high efficiency due to a definite separation of the field of redistribution from the environmental field and due to minimum absorption losses at the redistributing surface of such devices, and a favorable rejection of radiation impinging on the device from sources other than those intended for irradiating the device.
In attempting to provide radiation-redistributive devices having one or more of the features mentioned above, two approaches have been taken. One approach is purely empirical in nature and involves the evaluation of commercially available, inherently reflective, refractive, or diffuse materials to determine the utility thereof for a particular application. Exemplary of the projection screens developed through such an empirical approach are the volume and surface diffuser-type rear projection screens, and the aluminum foil front projection screens disclosed in the commonly assigned U.S. Pat. No. 3,408,132, high reflectance projection screens commercially available under the trademark Kodak Ektalite Projection Screen.
The second approach toward the provision of improved radiation-redistributive devices is analytical in nature, involving the derivation of mathematical expressions to define the contour which each elemental area of the redistributing surface must possess in order to achieve a desired redistribution of incident radiation, and the fabrication of an optical surfaces in accordance with such expressions. Lenticular projection screens and general lighting refractors are exemplary of such an analytical approach.
Notwithstanding the .[.approach.]. .Iadd.two different approaches .Iaddend.taken, radiation-redistributive devices heretofore proposed have not been totally satisfactory in all respects. Usually, certain desirable features are severely compromised to achieve other features which are deemed more desirable for a particular application. For instance, in the case of projection screens, several screens have been proposed having reflecting or refracting surfaces which, at least in theory, are capable of redistributing incident-image light in such a manner that the luminance of every elemental area on the screen surface is substantially constant throughout a predefined angular field of observation. Such screens, however, often suffer the disadvantages of being inefficient or wasteful of available image light and of being difficult, if not, for all practical purposes, totally impractical to manufacture in large quantities. See, for instance, the screens disclosed in U.S. Pat. No. 3,257,900 and U.S. Pat. No. 2,870,673. On the other hand, projection screens having highly efficient and readily manufacturable surfaces are often incapable of distributing incident image light uniformly and in a controlled manner, such surfaces commonly exhibiting "hot spots" or regions of non-uniform luminance.